The mixing of immiscible liquids to form a substantially uniform composition has been a classic problem for many industries. Examples of products where such mixtures are required are food products, cosmetics, toiletries, pharmaceuticals, paints and coatings. Such compositions extend from simple substances such as oil and water to complex substances. They may be characterized in many cases as non-polar hydrophobic materials combined with polar hydrophilic materials. These compositions are generally comprised of one of the phases in the form of discontinuous or discrete droplets dispersed through the other which forms the continuous phase, for example oil-in-water or water-in-oil. Critical to the uniformity of these compositions is the formation of droplets of very small size and a means of maintaining them in dispersed form, minimalizing any tendency to cause coalescence and thus separation of the phases.
The emulsion technology which has been created to address these difficulties is extensive and highly developed. Central to this technology is the use of such additives as surface active agents and detergents to form the droplets. The action of these additives is frequently supplemented by further additives such as protective colloids, gums, thickening agents and other classes of compounds which affect, alter and/or optimize the droplet size as well as the viscosity and stability of the emulsion.
The use of surface active agents requires special care for a number of reasons due to their chemical nature. During the manufacturing stages of the emulsion product, surface active agents raise considerations regarding their solubility, stability under varying conditions of temperature and pH, their frequent tendency to cause inversion of the phases, and their tendency to cause foaming, for example. Handling of surface active agents also requires special caution in many cases since they present potential health hazards upon inhalation and skin contact. Similar problems arise in end use of the finished products -- those requiring contact with human skin for utilitarian, industrial, cosmetic, toiletry or pharmaceutical purposes have produced stinging, irritation or sensitization for which the surface active agents have been implicated.
Oil-in-water type emulsions are often used for cosmetic and toiletry products. Such emulsions generally include mineral oils, vegetable oils, silicones, waxes, esters or the like as the internal phase, dispersed in aqueous solution, glycols or polyols.
Water-in-oil type emulsions encounter stability problems in much the same manner as the oil-in-water type. Such emulsions generally include aqueous solutions or glycols or polyols as the internal phase, dispersed in mineral oils, vegetable oils, silicones, waxes, esters or the like. Certain preparations involve the use of propylene glycol and/or propylene carbonate dispersed in petrolatum or liquid petrolatum. Emulsifying agents such as lanolin derivatives and cholesterol have been used. Unfortunately, the resulting emulsions are relatively unstable and tend to bleed and separate.